The present invention relates to a pulse oximeter for continuously measuring the oxygen saturation of the arterial blood of a subject in a bloodless manner by taking advantage of the difference in the characteristics of absorption by the organism of red light and infrared light emitting at different wavelengths. More particularly, the present invention relates to a pulse oximeter that permits simple correction of variations in the emission wavelength of the red LED that is provided in a probe.
A pulse oximeter is conventionally used to measure the oxygen saturation of arterial blood in a continuous and noninvasive manner. In actual measurement, the probe of the pulse oximeter is attached to the tip of a subject's finger or one of his earlobes and the living body of the subject is illuminated with red and infrared light at different wavelengths that are emitted from the probe at time intervals, and the oxygen saturation S of arterial blood is determined from the ratio .phi. between components of pulsation in absorbance as obtained from transmitted or reflected light for the two different wavelengths. A reference wavelength, say, 660 nm, is used for red light whereas a wavelength of 940 nm is typically used for infrared light. The probe contains in it two light-emitting diodes that emit at those two wavelengths and a single photodiode for light reception.
Suppose here that the absorbance of red light has a pulsating component .DELTA.A1 whereas the absorbance of infrared light has a pulsating component .DELTA.A2. The absorbance ratio .phi. for the two different wavelengths can be expressed by the following equation using .DELTA.A1 and .DELTA.A2: EQU .phi.=.DELTA.A1/.DELTA.A2 (A)
The oxygen saturation S is a function f of the absorbance ratio .phi. and is expressed as follows: EQU S=f(.phi.) (B)
The function f which correlates the oxygen saturation S to the absorbance ratio .phi. depends particularly on the wavelength of red light and the value of .phi. for the same value of oxygen saturation S varies with the wavelength of red light. FIG. 4 shows three characteristic curves for .phi. vs S at different wavelengths; curve U1 depicts the relationship at a wavelength of 660 nm; curve U2 plots the result obtained when the wavelength of red light is shifted to 650 nm; and curve U3 refers to the case where the wavelength is at 670 nm.
As is clear from those characteristic curves, if variations occur in the emission wavelength of the red LED contained in the probe, a certain correction must be made in order to measure the correct value of oxygen saturation S. This necessity presents no problem if the measurement is performed using the probe with which the pulse oximeter of interest is available since the necessary calibration has been completed for the oximeter. However, a problem arises if a different probe is to be attached to the main body of the oximeter for determining the oxygen saturation S.
Under the circumstances, there has been proposed an improved pulse oximeter of the type described in Unexamined Published Japanese Patent Application No. 64031/1984; the oximeter is so adapted that information on the emission wavelength of the red LED provided in the probe can be delivered from the probe to an external circuit to insure that the oxygen saturation S can be corrected by the main body of the oximeter on the basis of that wavelength information. Stated more specifically, the value of a certain wavelength is encoded and a resistor equivalent to the wavelength information for the red LED is mounted in the probe. The main body of the oximeter is so adapted that it is capable of reading the value of the resistor in terms of voltage, decoding the wavelength value, selecting an appropriate correction coefficient from a table that stores the values of red wavelength present for the main body and the correction coefficients necessary for calculating the oxygen saturation, and finally computing the correct value of oxygen saturation S.
FIG. 5 is a block diagram of a common pulse oximeter that uses the above-described prior art method of correction. A computing circuit 16 which is supplied with pulsating components of absorbances .DELTA.A1 and .DELTA.A2 for two wavelengths that have been detected with two detectors 13 and 14 and, using the values of .DELTA.A1 and .DELTA.A2, the absorbance ratio .phi. is computed in the computing circuit 16. The computed value of .phi. is sent to a computing circuit 17 in the next stage.
A probe 1 has a built-in resistor 20 that has encoded the information on the emission wavelength of a red LED 2. The value of the resistor 20 is read by a decoding portion 21 of the main body of the oximeter and converted to a value of wavelength, whereupon a correction coefficient corresponding to that value of wavelength is read out of a table 22 connected to the decoding portion 21. The table 22 is written in a ROM (read-only memory). The readout of the correction coefficient is set to the computing circuit 17, which uses both the value of absorbance ratio .phi. from the computing circuit 16 and the coefficient of correction to compute the oxygen saturation S that has been corrected for any variations in the emission wavelength of the red LED 2.
As described above, the conventional pulse oximeter which has the red LED 2 built in the probe 1 has seen it necessary to correct for any variations in the emission wavelength of that LED by equipping the probe 1 with the resistor 20 which encodes the value of said wavelength and which is equivalent to the wavelength information for the red LED. Another need has been to perform a decoding operation which consists of reading the value of the resistor 20 in the main body of the oximeter, thereby obtaining the value of the emission wavelength of the red LED. It has also been necessary to equip the main body with the table 22 storing those correction coefficients which are in one-to-one correspondence with the respective readout values of wavelength. All these factors have contributed to increasing the structural complexity of the pulse oximeter.
As for the table 22, a plurality of correction coefficients must be stored in consideration of the variations in the emission wavelength of the red LED 2 which is commercially available and, in a certain case, even those correction coefficients which are rarely used must be made available, thereby leading to complexity in the procedure for carrying out the decoding operation.
A further problem with the prior art pulse oximeter is that after an appropriate correction coefficient is selected on the basis of the wavelength information obtained from the probe 1, the correct value of oxygen saturation S must be determined using the selected coefficient but this only leads to complexity in the procedure for determining the oxygen saturation S.